<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Document</title>
	<style>
		.alpha {
			width: 300px;
			height: 300px;
			background: rgba(0, 0, 0, .2);
		}
	</style>
</head>
<body>

	<div class="alpha">
		哒哒哒
	</div>
</body>
</html>